Pleochroic dyes are dyes having the property that light absorption intensity depends on direction of the dye molecule. Namely, the absorption intensity of these dyes is maximized when the direction of absorption transition moment is parallel to the electric vector of light, and is minimized when it is perpendicular to the electric vector of light.
In the aforesaid liquid crystal display device, orientation of the dye molecules can be changed by utilizing the phenomenon that the orientation direction of liquid crystal molecules is changed by applying a voltage. Thus, it is possible to change the color of the liquid crystal display device by applying a voltage. The change of color is called "guest-host effect" taking the liquid crystal compound as host and the dye as guest.
Of the pleochroic dyes, those which have parallel dichroism show such properties that the direction of absorption transition moment of visible light is almost parallel to the longer axis of the molecule and, when dissolved as a guest molecule in a host liquid crystal, the dye molecules are aligned so that the longer axis of the dye molecule is in the same direction as the orientation direction of the axis of liquid crystal molecules.
For example, in a liquid crystal display device containing a liquid crystal composition comprising a pleochroic dye with parallel dichroism and a nematic liquid crystal compound with positive dielectric anisotropy and having been subjected to homogeneous orientation treatment, a homogeneous orientation is formed, wherein the longer axis of the dye molecules is aligned in parallel to the electrode plane similarly with the liquid crystal molecules. When white light traveling in a perpendicular direction to the electrode plane and polarized to the same direction as the orientation direction of the liquid crystal molecules travels through the liquid crystal layer in the above-described aligned state, the electric vector of the light becomes parallel with the dye molecules and a particular wavelength region is strongly absorbed and, as a result, the liquid crystal layer takes a strongly colored state. When a voltage is applied to the above-described liquid crystal layer, the longer axis of the dye molecule causes homeotropic orientation due to the positive dielectric anisotropy of the host liquid crystal. Thus, the longer axis of the dye molecules becomes perpendicular to the electric vector of incident white light, and hence the incident light is scarcely absorbed by the dye molecule and, as a result, the liquid crystal layer takes a weakly colored state.
The guest-host effect can be obtained not only by utilizing nematic liquid crystal compounds as described above but by utilizing, for example, smectic liquid crystal compounds or utilizing cholesteric-nematic phase change as well. In the case of utilizing phase transition, each molecule of a cholesteric liquid crystals in a device having been subjected to homogeneous orientation treatment takes a helical arrangement. When white light travels through the liquid crystal layer in a direction perpendicular to the electrode plane, a particular wavelength region of all polarization components is strongly absorbed by the dye molecules, because the dye molecules are existent in various directions. As a result, the liquid crystal layer becomes a strongly colored state.
Then, when the host cholesteric liquid crystals have positive dielectric anisotropy, application of a voltage to the liquid crystal layer in the above-described state causes homeotropic alignment of the longer axis of the dye molecule similarly with the liquid crystal molecule due to the relaxation of the helical arrangement of the liquid crystal molecules. As a result, the liquid crystal layer becomes a weakly colored state.
The pleochroic dyes as described above are required to possess: (1) a high "order parameter" (presented as S) in a host liquid crystal; (2) a sufficient solubility in a host liquid crystal; (3) a high stability against light, heat and electricity; and (4) a hue according to the end-use. Particularly, at least (1) and (2) are required for raising contrast of a resulting display device.
The order parameter S is defined by the equation (I) and is experimentally determined by the equation (II): ##EQU1## wherein the term of cos.sup.2 .theta. is timewise averaged, .theta. represents an angle between the absorption axis of the dye molecule and the orientation direction of the liquid crystal molecule, and A.parallel. and A.perp. represent the absorbances of the dye molecules for the light polarized parallel to and perpendicular to the orientation direction of the liquid crystal molecules, respectively.
With pleochroic dyes having parallel dichroism, the nearer the S value to 1 which is the theoretical maximum, the less the degree of residual color in a weakly colored state, which serves to realize high brightness, high contrast display.
An object of the present invention is to provide novel pleochroic dyes having parallel dichroism and capable of satisfying the aforesaid requirements (1), (2) and (3), a liquid crystal composition which can produce high contrast, high brightness display, and a display device containing the composition.
Relationship between the molecular structure of pleochroic dye and the various properties has not fully been clarified yet, and hence, it is quite difficult to select the pleochroic dye which has a desired hue and satisfies all requirements described hereinbefore based on knowledges about known materials.